The Food and Drug Administration (FDA) does not, as yet, have specific guidances for products containing nanoscale materials. As announced in the report issued by the FDA Nanotechnology Task Force (July 2007), however, there are recommendations to various centers within the FDA to develop guidances for industry. Regardless of the lack of explicit FDA guidances, there are therapeutics currently on the market containing nanoscale materials, and additional novel nanomaterial-containing therapeutics are being developed with the hopes of being submitted for regulatory review and approval. While, for the most part, these novel nanomaterial-containing products are being evaluated using the same regulatory requirements as products that do not contain nanomaterials, it is increasingly evident that at least in the area of characterization of nanomaterials used in drug products, there may be areas where special focus is needed. Specific areas include the validity of applying small molecule principles and methodologies to nanomaterial-containing products, the effects the nanomaterial will impart to the rest of the formulation (or vice versa), and how the physicochemical properties may be impacted by biological settings. Similarly, for safety evaluation, biodistribution studies will be at the core of any evaluation of products containing nanomaterials. These biodistribution studies will, in effect, be indicative of where the nanoparticles are traveling and possibly accumulating, therefore subjecting those sites to increased likelihood of toxicological effects. This chapter focuses on questions and considerations that may arise for sponsors during product characterization, as well as considerations for the appropriate design and conduct of in vivo toxicology studies. This chapter will also review how current FDA guidances apply to nanotherapeutics.This chapter reflects the current thinking and experience of the authors. However, this is not a policy document and should not be used in lieu of regulations, published FDA guidances, or direct discussions with the agency.